1. Field of the Invention
The present invention relates to a variable capacity swash plate type compressor able to be accommodated in an air-conditioner for a car, and more particularly, relates to a construction of a piston to be assembled in such a variable capacity swash plate type compressor.
2. Description of the Related Art
A typical conventional variable capacity swash plate type compressor is disclosed in Japanese Unexamined (Kokai) Patent publication No. 60-175783 published on Sep. 9, 1985, by the Japanese Patent Office.
FIG. 6 illustrates a compressor corresponding to the compressor of this publication. The compressor of FIG. 6 has a cylinder block 2 encased in a cylindrical shell 2a, and provided with a plurality of cylinder bores 21. The cylindrical shell 2a defines a closed crank chamber 31 therein, located axially in front of an inner end of the cylinder block. The crank chamber 31 is closed by a front housing 3 holding a radial bearing, to support an outer portion of a drive shaft 5, and rear ends of the cylinder block 2 and the cylindrical shell 2a are commonly closed by a rear housing 4 via a valve plate 41. The rear housing 4 is provided with an annularly extended suction chamber 42, and a cylindrical discharge chamber 44, which are communicated with the plurality of cylinder bores 21 of the cylinder block 2. The cylinder block 2 is centrally formed with a shaft bore, in which a radial bearing is seated, to rotatably support an inner end of the shaft 5. The drive shaft 5 has a central portion thereof on which a swash plate 6 is mounted, to be rotated with the shaft 5, about the axis of the drive shaft 5 within the crank chamber 31. The swash plate 6 is also able to wobble about an axis perpendicular to the axis of the drive shaft 5. An outer peripheral portion of the swash plate 6 is engaged, via spherical shoes 7, with pistons 1 slidably fitted in the cylinder bores 1 of the cylinder block. The cylinder block 2 is provided with a passageway 23 providing a fluid communication between the crank chamber 31 and the suction chamber 42 when a bellows-operated valve 8 is retracted from a passage-closed position to a passage-open position as shown in FIG. 6. The bellows-operated valve 8 operates in response to a change in a pressure differential between a preset pressure of the bellows and a suction pressure of the compressor.
When the drive shaft 5 is rotated together with swash plate 6, the pistons 1 reciprocate in the respective cylinder bores 21 to pump a refrigerant gas from the suction chamber 42 into the cylinder bores 21, and to discharge the refrigerant ga after compression from the cylinder bores 21 toward the discharge chamber 44. The volume or capacity of the compressed refrigerant gas discharged toward the discharge chamber 44 depends on a pressure prevailing in the crank chamber 31, which pressure is adjustably changed by the bellows-operated valve 8 controlling an extent of the fluid communication between the crank chamber 31 and the suction chamber 42. Namely, when the suction pressure is higher than the preset pressure of the bellows of the bellows-operated valve 8, the valve 8 is retracted to the passage-open position thereof, and therefore, the passageway 23 provides a required fluid communication between the crank chamber 31 and the suction chamber 42 to thus lower the pressure in the crank chamber 31. Accordingly, the stroke of each piston 1 and an inclination of the swash plate 6 from an erect position thereof are increased, to thereby increase the compression capacity of the compressor. Conversely, when the suction pressure is lower than the preset pressure of the bellows of the bellows-operated valve 8, the valve 8 is moved to the passage-closed position in which the passageway 23 does not provide a fluid communication between the crank chamber 31 and the suction chamber 42, and therefore, the pressure within the crank chamber 31 is raised to thereby decrease the stroke of each piston 1 and the inclination of the swash plate 6, and thus the compression capacity of the compressor is lowered.
During the compression operation of the compressor, a part of the compressed refrigerant gas always leaks from the cylinder bores 21 into the crank chamber 31, and therefore, this leaking refrigerant gas is able to act as a lubricant for all moving parts of the compressor, i.e., the swash plate 6 and the shoes 7.
With the above-described variable capacity swash plate type compressor, the pistons 1 preferably has a light weight, from the viewpoint of a reduction of a load applied to a drive source of the compressor, e.g., a car engine. Accordingly, as shown in FIG. 6, a main body of each piston 1, i.e., a cylindrical portion of each piston 1 reciprocating in the cylinder bore 21, is formed with an open space therein, and a protrusion thereof is axially extended from the main body to be engaged with a radial aperture at the periphery of the swash plate 6 via the shoes 7. Nevertheless, since the reciprocatory motion of each piston 1 in the cylinder bore 21 is caused by the rotating motion of the swash plate 6, an unfavorable force is applied by the rotating swash plate 6 on each piston 1, in a direction substantially corresponding to the direction of rotation of the swash plate 6. Accordingly, the axial protrusion of the piston engaged with the swash plate 6 and the shoes 7 must be able to physically withstand such an unfavorable force. Taking this into consideration, an improvement of the piston 1 shown in FIG. 6 can be made by constructing a reciprocating piston in a manner such that a main body thereof has the shape of a closed hollow thin wall cylinder. Namely, the closed hollow cylinder construction of the main body of each piston enable the providing of an axial protrusion thereof from the main body, physically able to withstand the afore-mentioned unfavorable force acting on the piston, and having a sufficiently light weight.
Nevertheless, the production of the piston having the closed hollow cylindrical body requires a particular fabrication such that at least two separate cylindrical hollow elements are joined together by welding, and therefore, when joined, the closed hollow cylindrical body of the piston sealingly contains air at an approximately atmospheric pressure therein.
When the above-mentioned type of pistons are accommodated in a variable capacity swash plate type compressor, and when a refrigerant gas at a high pressure 4 to 5 times higher than the atmospheric pressure is filled in a refrigerating circuit incorporating the compressor therein, a large pressure differential must appear between the pressure of the refrigerant gas and the pressure within the main body of each piston. Furthermore, a larger pressure differential is produced between a pressure in the cylinder bores, wherein an increase and a decrease of the pressure are repeated in response to the compression and suction of the refrigerant gas, and a pressure of the interior of each piston during the reciprocating operation of the pistons. As a result, a large variable pressure acts on the respective pistons and causes a deformation of the main body of each piston having a thin wall, during the reciprocation of the pistons. Further, when the pistons are subjected to such a large variable pressure, the welded portion of the main body of the piston is apt to be broken, and if broken, the air emerges from the interior of the main body of the piston and is mixed with the refrigerant gas, and such a mixture of air and refrigerant gas has an adverse affect on the operation of the compressor. Therefore, when a piston having a closed hollow cylindrical body is employed in a variable capacity type refrigerant compressor, the wall thickness of the main body of the piston cannot be thinner than a given limit, and thus a sufficient reduction in the weight of the piston cannot be obtained.